AP-G84/04 Best practice in road use data collection, analysis ... - WIM

Accessed by AR - ARRB TRANSPORT RESEARCH on 04 Feb 2005
Austroads 2004
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Best Practices in Road Use Data Collection, Analysis and Reporting
(c) Crossfall - ideally the crossfall shall be between 2% and 3% with a minimum of 2%. The
designer needs to balance the needs of CULWAY, i.e. as flat and smooth as possible against
the engineering necessity of adequately draining the road.
(d) Open speed section - the speed of the traffic may also impact CULWAY results. Normal
highway speeds are desirable for optimal operation. Slow speed, in particular speeds of less
than 70 km/h may affect accuracy. Vehicles travelling below 30 km/h will probably be
overweighed. The presence of nearby intersecting roads or any other road geometry that
causes traffic acceleration or deceleration over the CULWAY is undesirable.
(e) Culvert cover - a section of fill providing adequate cover for the culvert after ensuring a
culvert of minimum height 600 mm can be placed above natural surface.
(f) Culvert to be dry - the CULWAY should be located in a section where the fall is sufficient to
achieve essentially what will be a dry culvert or at least will ensure that any water will drain
away quickly from the CULWAY.
6.4.2 Time-Dependent Drift between Calibrations
It has been known for some time that the CULWAY data is affected by time-of-day and week-ofyear
(Lim 1992). The variation or drift is dependent on site and environment conditions. The
strength of the bituminous surface on top of a culvert box is affected by temperature, and how well
air and water are drained through the surface.
Grundy (2002) reported the study of two CULWAY sites in Victoria, making use of the property that
the steering axles mass (SAM) of an articulated six-axle is the least affected by temperature and
drainage conditions. This is because there is no direct load on the steering axle (
Figure 14). It is therefore possible to use this constancy of average SAM to correct for diurnal and
seasonal drift in WIM sensitivity.
The results are shown in Figure 15 for the diurnal effect of hour for the sites – Western Ring Road
in Melbourne and Yarra Glen just outside Melbourne. Figure 16 shows the seasonal variation over
six months (July to December). The variation can be substantial in the case of the Western Ring
Road. The correction factor varied from 0.93 (winter) to 1.1 (summer). The Yarra Glen site
showed less variation, possibly due to the use of a more traditional seal whereas the more porous
surface of the Western Ring Road exhibited more variation with temperature and drainage.
A CULWAY system has its own temperature-compensating mechanism in its electronic hardware
and provides reasonable results with acceptable accuracy levels but does not consider
temperature/climatic variations of the pavement on top of the culvert box. It is therefore good
practice to pay attention to site conditions and how seasonal changes in climate could affect a WIM
installation. Some RAs already allow for seasonal and diurnal drifts of CULWAY.